Titanium-A armor is the most common form of battle plating used by the UNSC on its warships. It is composed of high-grade Titanium-50 that is specially strengthened at the molecular level, which implies alloying with other elements. It is usually greyish in color.
Titanium-A has numerous uses throughout the UNSC due largely to titanium being lightweight and extremely strong, with a high tolerance to heat. It took a new found importance after the beginning of the Human-Covenant War: To contend with the Covenant's plasma weaponry, Humans needed armor that would withstand the heat of the directed plasma. Despite its strength and resistance to heat, however, the Titanium-A plating is usually boiled away within seconds, and does not effectively dissipate the plasma.
Its primary use is armor for ships. UNSC frigates have sixty centimeters of plating on their hull, and UNSC destroyers have up to two meters of plating. Marines sometimes wear thin Titanium-A armor suits. Later in the war, these suits were replaced by ceramic armor suits, such as those used by the Orbital Drop Shock Troopers.
The UNSC uses Titanium-A for construction of Bases as well, to make them less susceptible to destruction. Places such as HIGHCOM Facility Bravo-6, CASTLE Base and Camp Hathcock used Titanium-A armor in their superstructures.
 Titanium-A3 armor
Titanium-A3 armor is a variant of Titanium-A battle plate. Like Titanium-A, it is chemically altered on the molecular level to produce a stronger plate for ship hulls. It has only been noted to be used on the UNSC Infinity. One can assume that this armor is much stronger than its older variant by looking at the strength, power, importance, and notability of the Infinity over other ships.